1. Field of the Invention
The present invention relates to a coal-water slurry producing system having a mill for mixing a solid fuel such as coal and petroleum coke with water and grinding the same to produce a slurry. More particularly, the present invention relates to a coal-water slurry producing system incorporating an improved filtering apparatus for removing large solid particles in a slurry discharged from a mill and a ball mill used for said system.
2. Related Art Statement
A slurry of coal and water in which ground coal is mixed with water with the addition of additives such as a surface active agent and is made into a slurry is easy to handle in terms of transportion and storage. Therefore, research has been under way with regard to the use of such a coal-water slurry as a fuel for combustion apparatuses such as boilers.
It is required that a coal-water slurry used as a boiler fuel is such that the concentration of coal is about 60 wt. % or above and that the particle size is sufficiently small to allow 60-90 wt. % of the particles to pass through a 200-mesh screen. In order to obtain such a coal-water slurry, one method has been adopted which involves grinding coal with water and additives by means of a continuous wet-type ball mill.
In a coal-water slurry producing system having a continuous wet-type ball mill, coarse coal particles which have not been ground well are mixed in the coal-water slurry discharged from the mill. If the slurry with such coarse particles mixed therein is sent to a strainer, the load applied to the metal screen in the strainer becomes excessively large, so that the strainer becomes frequently blocked and may be damaged by the excessive load even if the strainer is designed with substantial leeway in terms of the filtering capacity of the metal screen. Since such a strainer per se is expensive, damage thereof entails a substantial loss. In addition, damage of the strainer involves the problem of leading to a shutdown of the overall coal-water slurry producing system.
Meanwhile, there are various factors that generally affect the performance of a continuous-type ball mill including the ball charge, the ball diameter, the mill rotating speed, the outlet structure of the mill and the dimensional ratio of the mill. As for the ball charge, one of 35 to 45% is generally used (refer to "Mineral Processing Plant Design" chapter 12, SME/AIME, 1979 and "Process Engineering of Size Reduction: Ball Milling L. G. Austin et al., AIME, 1984). Regarding the ball diameter, a mixture of various sizes of balls are employed which are best suited for obtaining a distribution of product particle sizes from a given distribution of raw-material particle sizes. As for the mill rotational speed, the speed used is one that is equivalent to approximately 65 to 80% of the critical speed (i.e., a speed at which centrifugal force and gravity are balanced, and at which balls rotate with the mill along the inner wall of the mill). With respect to the outlet structure of the mill, the diameter of the outlet is important. In addition, the ratio of the mill outlet diameter Dd to the mill inner diameter D, i.e., Dd/D, is set at approximately 0.2 to 0.3 on the basis of a ball space filling rate U of particles inside the mill, this rate having been proposed from the viewpoint of grinding efficiency (refer to "Process Engineering of Size Reduction: Ball Milling"). The ratio of mill dimensions, i.e., the ratio of the mill length L to the mill inner diameter D, i.e., L/D, is generally set at 2 to 3. However, a ball mill which is operated under these conditions suffer from a high rate of power consumption. For instance, in an experiment conducted by the present inventors under the following conditions, the cost of power consumption was 13.3% of the coal cost.
Ball mill: .phi.300.times.900L (L/D=2.5) PA1 Rotational speed: 70% of the critical speed PA1 Ball diameter: 40-17 mm PA1 Ball charge: 35% PA1 Ball outlet opening ratio: Dd/D=0.28 PA1 Feeding rate of coal: 6 kg/h PA1 Coal concentration: 62.5% PA1 Amount of surface active agent added: 0.7 wt. % with respect to coal PA1 Amount of pH adjusting agent added: 0.1 wt. % with respect to coal PA1 Result: The specific energy consumption for grinding was about 87 kWh/t.
If the unit price of electric power is assumed to be 23/kWh, the power cost becomes 20,001/ton. If the unit price of coal is assumed to be 15,000/ton, the power cost is equivalent to about 13.3% of the coal cost.